Add SQL queries profiler in Debug zone with backtrace of call and EXPLAIN hovers

This implements FR #31087 (only active in debug-mode).
Pretty handy in improving SQL queries and database scheme for speed.
Made a quick hack for my JAB13 talk, and re-did it on my return trip from JAB13 cleanly.

http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=31087

Feel free to improve upon it by issuing pull requests on my fork (on my branch mysqlprofiling).